c# - 在运行时更改表达式委托(delegate)主体？

Question

我有这段代码生成一个将 myNumber 乘以 5 的委托(delegate)

ParameterExpression numParam = Expression.Parameter(typeof(int), "num");
ConstantExpression five = Expression.Constant(5, typeof(int));
BinaryExpression numMultiply = Expression.Multiply(numParam,five);

让我们创建委托(delegate):

Expression<Func<int, int>> lambda1 =
    Expression.Lambda<Func<int, int>>(
        numMultiply,
        new ParameterExpression[] { numParam });
Console.Write(lambda1.Compile()(4));

现在假设我想将此表达式树更改为 Add 而不是 Multiply
这是新行:

BinaryExpression numAdd = Expression.Add(numParam,five);

但是我怎样才能更改 lambda1 以便它现在将使用 numAdd 而不是 multiply ？

Answer 1

你只需构建一个新的，然后编译它。

Expression<Func<int, int>> lambda1 =
     Expression.Lambda<Func<int, int>>(
         numAdd,
         new ParameterExpression[] { numParam });

来自MSDN page :

Expression trees should be immutable. This means that if you want to modify an expression tree, you must construct a new expression tree by copying the existing one and replacing nodes in it. You can use an expression tree visitor to traverse the existing expression tree.

“应该是”这个短语有点奇怪，但是当您查看 API 时，您会发现所有相关属性(Body、Left、Right)都是只读的。